In manufacturing semiconductor devices, a vertical type of thermal processing apparatus is used, which is capable of thermally processing a large number of objects to be processed, such as semiconductor wafers (product wafers), at a time (large-amount batch process). For example, as shown in JP Laid-Open Publication No. 2000-150400 and JP Publication No. 2681055, a vertical type of thermal processing apparatus comprises a thermal processing furnace having a furnace opening at a lower portion thereof. A boat holding a large number of wafers (objects to be processed) in a tier-like manner in a vertical direction is contained inside the thermal processing furnace through the furnace opening. A lid supporting the boat can close the furnace opening. The furnace opening is connected to a transfer chamber. In the transfer chamber, an elevating mechanism is provided to move up and down the lid in order to load and unload the boat into and out from the thermal processing furnace. In the transfer chamber, provided are a boat placing portion on which two boats can be placed, a boat changing mechanism (boat changer) that can change boats between the boat placing portion and the lid, and a transferring mechanism that can transfer wafers between a conveying container (cassette, carrier, FOUP or the like) that can contain a plurality of wafers and a boat placed on the boat placing portion.
According to the above vertical type of thermal processing apparatus, the two boats can be used. Thus, while one boat is placed on the lid that can open and close the furnace opening of the thermal processing furnace, the boat is loaded into the thermal processing furnace, and wafers held on the boat are thermally processed, wafers can be transferred with respect to the other boat placed on the boat placing portion. Thus, throughput can be improved.
However, the boat changing mechanism is necessary in the above vertical type of thermal processing apparatus. Thus, the apparatus has to be large and hence cost thereof has to be large. In addition, if an earthquake occurs in changing the boats, the boats may be swung and fallen down, so that the wafers and the boats may be broken. Furthermore, when the two boats are used, depending on minute difference between boats' shapes and difference between boats' operating situations, film-forming property to the wafers may be not uniform. In addition, in this case, it is difficult to manage total thickness of films on the boats.
In addition, if a small-amount batch process is conducted wherein the number of product wafers is small and the number of dummy wafers (Fill Dummy) is large, it is necessary to store (stock) a conveying container for the dummy wafers in the transferring chamber. Thus, a space and a facility for the conveying container are necessary, which enlarges the apparatus and increases cost of the apparatus.